Method and apparatus for third heart sound detection

ABSTRACT

A cardiac rhythm management system includes a heart sound detector providing for detection of the third heart sounds (S 3 ). An implantable sensor such as an accelerometer or a microphone senses an acoustic signal indicative heart sounds including the second heart sounds (S 2 ) and S 3 . The heart sound detector detects occurrences of S 2  and starts S 3  detection windows each after a predetermined delay after a detected occurrence of S 2 . The occurrences of S 3  are then detected from the acoustic signal within the S 3  detection windows.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/746,853, filed Dec. 24, 2003, now issued as U.S. Pat. No. 7,431,699,which is hereby incorporated by reference in its entirety.

This application is related to commonly assigned U.S. patent applicationSer. No. 10/746,874, entitled “A THIRD HEART SOUND ACTIVITY INDEX FORHEART FAILURE MONITORING,” filed on Dec. 24, 2003, now U.S. Pat. No.7,115,096, U.S. patent application Ser. No. 10/334,694, entitled “METHODAND APPARATUS FOR MONITORING OF DIASTOLIC HEMODYNAMICS,” filed Dec. 30,2002, now issued as U.S. Pat. No. 7,972,275, and U.S. patent applicationSer. No. 10/307,896, “PHONOCARDIOGRAPHIC IMAGE-BASED ATRIOVENTRICULARDELAY OPTIMIZATION,” filed Dec. 12, 2002, now issued as U.S. Pat. No.7,123,962, all assigned to Cardiac Pacemakers, Inc., which are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

This document relates generally to cardiac rhythm management systems andparticularly, but not by way of limitation, to such a system sensing andanalyzing heart sounds for monitoring, diagnosis, and therapy control.

BACKGROUND

The heart is the center of a person's circulatory system. It includes acomplex electromechanical system performing two major pumping functions.The left portions of the heart draw oxygenated blood from the lungs andpump it to the organs of the body to provide the organs with theirmetabolic needs for oxygen. The right portions of the heart drawdeoxygenated blood from the organs and pump it into the lungs where theblood gets oxygenated. These mechanical pumping functions areaccomplished by contractions of the myocardium (heart muscles). In anormal heart, the sinoatrial (SA) node, the heart's natural pacemaker,generates electrical impulses, called action potentials, that propagatethrough an electrical conduction system to various regions of the heartto excite myocardial tissues in these regions. Coordinated delays in thepropagations of the action potentials in a normal electrical conductionsystem cause the muscles in various regions of the heart to contract insynchrony such that the pumping functions are performed efficiently. Thenormal pumping functions of the heart, or the normal hemodynamicperformance, require a normal electrical system to generate the actionpotentials and deliver them to designated portions of the myocardiumwith proper timing, a normal myocardium capable of contracting withsufficient strength, and a normal electromechanical association suchthat all regions of the heart are excitable by the action potentials.

Electrocardiography (ECG) is known to indicate the functions of theelectrical system by allowing monitoring of the action potentials atvarious portions of the heart. Heart sounds, or generally energiesresulted from the heart's mechanical vibrations, indicate the heart'smechanical activities. Measurements performed with simultaneouslyrecorded ECG and heart sounds provide for quantitative indications ofthe electromechanical association.

One type of heart sound, known as the third heart sound, or S3, is knownas an indication of heart failure. A heart failure patient suffers froman abnormal electrical conduction system with excessive conductiondelays and deteriorated heart muscles that result in asynchronous andweak heart contraction, and hence, reduced pumping efficiency, or poorhemodynamic performance. While the ECG of a heart failure patient mayshow excessive delays and/or blockages in portions of the electricalconduction system, S3 indicates his or her heart's abnormal mechanicalfunctions. For example, an increase in S3 activity is known to be anindication of elevated filing pressures, which may result in a state ofdecompensated heart failure. Additionally, S3 amplitude is also relatedto filing pressures of the left ventricle during diastole. The pitch, orfundamental frequency, of S3 is related to ventricular stiffness anddimension. Chronic changes in S3 amplitude are correlated to leftventricular chamber stiffness and degree of restrictive filling. Suchparameters indicate abnormal cardiac conditions, including degrees ofseverity, and need of appropriate therapies.

For these and other reasons, there is a need for a system providing forS3 detection and analysis.

SUMMARY

A cardiac rhythm management system includes a heart sound detectorproviding for detection of the third heart sounds (S3). An implantablesensor such as an accelerometer or a microphone senses an acousticsignal indicative heart sounds including the second heart sounds (S2)and S3. The heart sound detector detects occurrences of S2 and starts S3detection windows each after a predetermined delay after a detectedoccurrence of S2. The occurrences of S3 are then detected from theacoustic signal within the S3 detection windows.

In one embodiment, a heart sound detection system includes a cardiacsignal input, an acoustic signal input, and a heart sound detector. Thecardiac signal input receives a cardiac signal indicative of ventricularevents. The acoustic signal input receives an acoustic signal indicativeof at least S2 and S3. The heart sound detector includes an S2 windowgenerator, an S2 detector, an S3 window generator, and an S3 detector.The S2 window generator generates an S2 window after a predetermineddelay starting with each of the ventricular events. The S2 detectordetects S2 during the S2 windows. The S3 window generator generates anS3 window after a predetermined delay starting with each of the detectedS2. The S3 detector detects S3 during the S3 windows.

In one embodiment, a heart sound detection method provides for S3detection. A cardiac signal indicative of ventricular events and anacoustic signal indicative of at least S2 and S3 are received. S2 aredetected by comparing the acoustic signal to an S2 threshold. An S3window is generated after a first predetermined delay starting with eachof the detected S2. S3 are detected during the S3 window by comparingthe acoustic signal to a dynamically adjustable S3 threshold.

In one embodiment, an implantable cardiac rhythm management systemincludes an implantable lead, an implantable acoustic sensor, and animplantable medical device. The implantable lead is used for sensing acardiac signal indicative of ventricular events. The implantableacoustic sensor is used to sense an acoustic signal indicative of atleast S2 and S3. The implantable medical device includes a cardiacsignal input to receive the cardiac signal, an acoustic signal input toreceive the acoustic signal, and a heart sound detector to detect S2 andS3. The heart sound detector includes an S2 window generator, an S2detector, an S3 window generator, and an S3 detector. The S2 windowgenerator generates an S2 window after a predetermined delay startingwith each of the ventricular events. The S2 detector detects S2 duringthe S2 windows. The S3 window generator generate an S3 window afteranother predetermined delay starting with each of the detected S2. TheS3 detector to detect S3 during the S3 windows.

This Summary is an overview of some of the teachings of the presentapplication and not intended to be an exclusive or exhaustive treatmentof the present subject matter. Further details about the present subjectmatter are found in the detailed description and appended claims. Otheraspects of the invention will be apparent to persons skilled in the artupon reading and understanding the following detailed description andviewing the drawings that form a part thereof, each of which are not tobe taken in a limiting sense. The scope of the present invention isdefined by the appended claims and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsdescribe similar components throughout the several views. The drawingsillustrate generally, by way of example, but not by way of limitation,various embodiments discussed in the present document.

FIG. 1 is a block diagram illustrating an embodiment of a heart soundprocessing system for trending an S3 index.

FIG. 2 is a block diagram illustrating an embodiment of a heart sounddetector.

FIG. 3 is an illustration of an embodiment of a method for detectingselected heart sounds.

FIG. 4 is a flow chart illustrating an embodiment of a method fortrending the S3 index using the system of FIG. 1.

FIG. 5 is a flow chart illustrating an embodiment of a method fordetecting S3 using the heart sound detector of FIG. 2.

FIG. 6 is an illustration of an embodiment of portions of a cardiacrhythm management system incorporating the heart sound processing systemof FIG. 1, and portions of an environment in which it is used.

FIG. 7 is a block diagram illustrating a specific embodiment of portionsof the cardiac rhythm management system of FIG. 6.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description provides examples,and the scope of the present invention is defined by the appended claimsand their equivalents.

It should be noted that references to “an”, “one”, or “various”embodiments in this disclosure are not necessarily to the sameembodiment, and such references contemplate more than one embodiment.

This document discusses, among other things, a cardiac rhythm managementsystem monitoring and analyzing heart sounds, particularly the thirdheart sounds (S3), that are indicative of a heart's mechanical eventsrelated to the heart's pumping functions and hemodynamic performance toallow, among other things, diagnosis of cardiac conditions and selectionof therapies treating the cardiac conditions. The cardiac rhythmmanagement systems include systems having, for example, pacemakers,cardioverter/defibrillators, pacemaker/defibrillators, cardiacresynchronization therapy (CRT) devices, and cardiac remodeling controldevices. However, it is to be understood that the present methods andapparatuses may be employed in other types of medical devices,including, but not being limited to, drug delivery systems and varioustypes of cardiac monitoring devices.

More particularly, the cardiac rhythm management system discussed inthis document generates and trends an S3 index indicative of S3activity. The S3 index (or prevalence) is a ratio of the number of heartbeats during which S3 are detected (“S3 beats”) to the number of all theheart beats. Because the S3 activity varies throughout the day, the S3beats are counted for a plurality of measurement sessions distributedover a measurement period. The S3 index is then calculated for themeasurement period and trended over multiple measurement periods. Atrend of the S3 index provides for an indication of heart failure. Forexample, an increase in the trend of the S3 index may be indicative ofabnormally restrictive filling and elevated filling pressures that leadto edema.

While this document particularly relates to S3, other hearts sounds arealso detected and/or analyzed for S3 detection and other purposes. Knownand studied heart sounds include the “first heart sound” or S1, the“second heart sound” or S2, the “third heart sound” or S3, the “fourthheart sound” or S4, and their various sub-components. S1 is known to beindicative of, among other things, mitral valve closure, tricuspid valveclosure, and aortic valve opening. S2 is known to be indicative of,among other things, aortic valve closure and pulmonary valve closure. S3is known to be a ventricular diastolic filling sound often indicative ofcertain pathological conditions including heart failure. S4 is known tobe a ventricular diastolic filling sound resulted from atrialcontraction and is usually indicative of pathological conditions. Theterm “heart sound” hereinafter refers to any heart sound (e.g., S1) andany components thereof (e.g., M1 component of S1, indicative of Mitralvalve closure).

Throughout this document, “heart sound” includes audible and inaudiblemechanical vibrations caused by cardiac activity that can be sensed withan accelerometer. Accordingly, when a mechanical sensor such as anaccelerometer is used to sense the heart sounds, the scope of energyincluded in the sensed “acoustic signal” extends to energies associatedwith such mechanical vibrations. Unless noted otherwise, S1 refers tothe first heart sound, S2 refers to the second heart sound, S3 refers tothe third heart sound, and S4 refers to the fourth heart sounds, each asa heart sound type, or as one or more occurrences of the correspondingtype heart sounds, depending on the context. A “heart beat” includes acardiac cycle. An “S3 beat” includes a cardiac cycle during which S3 isdetected. An “S3 index,” also referred to as an “S3 ratio,” includes aratio of the number of the S3 beats to the number of the total hearthearts, both detected during the same time period.

Throughout this document, a “user” includes a physician or othercaregiver who examines and/or treats a patient using one or more of themethods and apparatuses reported in the present document.

FIG. 1 is a block diagram illustrating an embodiment of a heart soundprocessing system 100 for trending the S3 index. Heart sound processingsystem 100 trends the S3 index by calculating an average ratio of thenumber of S3 beats to the total number of heart beats counted overpredetermined measurement sessions. It includes a cardiac signal input101, a beat counter 103, an acoustic signal input 104, a heart sounddetector 105, a heart sound counter 106, an activity signal input 107,an activity detector 108, a measurement timer 109, a measurement delaytimer 110, and a heart sound analyzer 112.

Cardiac signal input 101 receives cardiac signals indicative of cardiacevents such as atrial and ventricular contractions. The cardiac signalsinclude electrocardiographic (ECG) signals and/or event markersindicative of occurrences and timing of the cardiac events. In oneembodiment, cardiac signal input 101 receives at least one ventricularelectrogram (intracardiac ECG) acquired through at least oneintracardiac electrode coupled to an implantable device. The ventricularelectrogram is indicative of intrinsic ventricular events (sensedR-waves) and paced ventricular events (ventricular pacing pulses and/orpaced R-waves). In another embodiment, cardiac signal input 101 receivesat least one signal including event markers indicative of occurrencesand timing of intrinsic and paced ventricular events. The intrinsic andpaced ventricular events provide for heart beat detection and countingand serve as a timing reference for heart sound detections, as discussedbelow with reference to FIGS. 2 and 3.

Beat counter 103 counts the number of detected heart beats. In oneembodiment, beat counter 103 counts one beat for each intrinsic or pacedventricular event.

Acoustic signal input 104 receives an acoustic signal indicative of oneor more types of heart sounds including at least S3. In one embodiment,the acoustic signal is acquired by a microphone sensing acousticenergies generated by mechanical vibrations of the heart. In anotherembodiment, the acoustic signal is acquired by an accelerometer sensingenergies associated with the mechanical vibrations of the heart,including both audible and inaudible energies. In one furtherembodiment, the acoustic signal is a rectified and low-pass filteredsignal, such as an acoustic signal 340 illustrated in FIG. 3.

Heart sound detector 105 detects at least S3 from the acoustic signalreceived by acoustic signal input 104. In one embodiment, heart sounddetector also detects S1 and/or S2 during the process of detecting S3.Heart sound detector 105 is further discussed below with reference toFIG. 2.

Heart sound counter 106 counts the number of S3 beats. During eachcounted heart beat, if heart sound detector 105 detects an S3, heartsound counter 106 counts one S3 beat.

Activity signal input 107 detects a physical activity level of a personfrom whom the heart sounds are detected. When the acoustic signal isacquired by an accelerometer, physical activities may be sensed as anoise because the accelerometer senses both the heart's vibrations andthe person's gross bodily movements. The physical activity level thusdetermines whether the heart sounds can be reliably detected, i.e.,without detecting the person's gross activities as heart sounds.

Activity detector 108 determines whether the physical activity levelexceeds a threshold above which heart sound detection is consideredunreliable. In one embodiment, activity detector 108 includes acomparator to compare the physical activity level to a predeterminedthreshold. Activity detector 108 generates a permission signal when thephysical activity level is below the threshold.

Measurement timer 109 starts and times measurement sessions during whichthe heart beats and the S3 beats are counted. In one embodiment,measurement timer 109 includes a measurement period timer and ameasurement timer. Each measurement period includes a plurality ofmeasurement sessions each start at prescheduled times. In oneembodiment, adjacent measurement periods overlap. For example,measurement period M includes predetermined measurement sessions N, N+1,and N+2; measurement period M+1 includes predetermined measurementsessions N+1, N+2, and N+3; and so on. In another embodiment,measurement periods do not overlap. For example, measurement period Mincludes measurement sessions N, N+1, and N+2; measurement period M+1includes measurement sessions N+3, N+4, and N+5; and so on. In oneembodiment, a user determines and programs the timing of the measurementperiods and the timing of the measurement sessions. In one specificembodiment, the user makes the determination based on the need and theconditions of a particular patient. In one embodiment, measurement timer109 includes a measurement period clock to time a predetermined timeinterval as one measurement period. In another embodiment, measurementtimer 109 includes a measurement period counter to count a predeterminednumber of measurement sessions as a measurement period. In oneembodiment, measurement timer 109 includes a measurement session clockto time a predetermined time interval as one measurement session. Inanother embodiment, measurement timer 109 includes a measurement sessioncounter to count a predetermined number of heart beats as onemeasurement session. Measurement timer 109 starts a measurement sessiononly when the output of activity detector 108 represents a permissionsignal. In one specific embodiment, the measurement period is one dayand includes about 4 to 48 measurement sessions, depending on thepatient's circumstances, and each measurement session lasts for about 10to 30 beats.

Measurement delay timer 110 delays the start of a measurement sessionwhen a scheduled measurement session is prevented from starting by anabsence of the permission signal. In one embodiment, measurement delaytimer 110 delays the start of a measurement session until the permissionsignal is given by activity detector 108. In another embodiment,measurement delay timer110 checks the presence of the permission signalperiodically with a predetermined delay period, and allows the start ofa measurement session when the permission signal is present at the endof one of the predetermined delay periods.

Heart sound analyzer 112 generates a trend of S3 index based on a ratioof the number of S3 beats to the number of total heart beats countedduring a predetermined time period (the S3 index or S3 ratio). In oneembodiment, the ratio is expressed as a percentage (the S3 percentage).Heart sound analyzer 112 includes a ratio calculator 113, an averagingmodule 114, and a trend generator 115. In one embodiment, ratiocalculator 113 receives the number of the S3 beats and the number of theheart beats counted during each measurement session and calculate the S3ratio for each measurement session, resulting in an S3 session ratios(i.e., S3 indexes for the measurement sessions). In one specificembodiment, ratio calculator 113 includes a percentage calculator tocalculate the S3 percentage for the measurement session, resulting in S3session percentages. In another embodiment, ratio calculator 113estimates the S3 ratio for each measurement session based on some butnot all the beats (such as every other beat), for example, whendetecting and/or counting each and every beat and/or S3 beat isimpractical. Averaging module 114 calculates an average of the S3session ratios over each measurement period to result in S3 periodicratios (i.e., S3 indexes for the measurement periods). Trend generator115 generates the trend of S3 index as a moving S3 periodic ratio over aplurality of the measurement periods, where each S3 periodic ratio is anaverage of the S3 session ratios for one of the measurement periods. Inan alternative embodiment, ratio calculator 113 receives the number ofthe S3 beats and the number of the heart beats counted during eachmeasurement period and calculate the S3 ratio for that measurementperiod, directly resulting in S3 periodic ratios. Trend generator 115generates the trend of S3 index as a moving S3 periodic ratio over aplurality of the measurement periods. In one embodiment, trend generator115 generates the trend of S3 index as a plot of daily S3 ratios overmultiple days.

FIG. 2 is a block diagram illustrating an embodiment of heart sounddetector 105. Heart sound detector 105 detects S3 by using the methodillustrated in FIG. 3. FIG. 3 shows an acoustic signal 340 indicative ofan S1 occurrence 341, an S2 occurrence 342, and an S3 occurrence 343during one cardiac cycle. In one embodiment, acoustic signal 340represents an output signal of an envelope detector that rectifies andlow-pass filters a sensor signal indicative of heart sounds. Acousticinput 104 receives acoustic signal 340. In one embodiment, the S3detection method employed by heart sound detector 105 provides fordetection of at least S2 and S3. Detection of S1 is required for thedetection of S3 when the amplitude and/or energy of S1 are needed toadjust one or more detection thresholds including an S2 threshold 346and an S3 threshold 347.

S2 and S3 detections are repeated for each cardiac cycle when the heartsounds are being detected. S2 detection includes comparing the amplitudeof acoustic signal 340 to S2 threshold 346 during an S2 window 354. S2window has a predetermined temporal relationship with a ventricular (V)event detection 349, such as an R-wave or a delivery of a ventricularpacing pulse. As illustrated in FIG. 3, V event detection 349 starts apredetermined S2 window delay 355. S2 window 354 starts when S2 windowdelay 355 expires. S2 detection occurs when the amplitude of acousticsignal 340 exceeds S2 threshold 346 during S2 window 354. S2 windowdelay 355 and the duration of S2 window 354 are programmed on apatient-by-patient basis. In one embodiment, the timing of S2 detection350 is empirically estimated for each individual patient and dynamicallyadjusted based on the patient's heart rate. In one specific example, thetime interval between V event detection 349 and S2 detection 350,T_(V-S2), is estimated by: T_(V-S2)=0.500−0.002 HR seconds, where HR isheart rate in beats per minute, and 0.500 seconds is empirically derivedfor the individual patient. Then, the duration of S2 window 354 isempirically derived for that patient, and S2 window 354 is centered atthe estimated time for S2 detection 350, i.e., end of T_(V-S2). S2window delay is, therefore, T_(V-S2) minus a half of the duration of S2window 354.

S3 detection includes comparing the amplitude of acoustic signal 340 toS3 threshold 347 during an S3 window 356. S3 window has a predeterminedtemporal relationship with S2 detection 350. An S3 window delay 357starts with S2 detection 350. S3 window 356 starts when S3 window delay357 expires. S3 detection occurs when the amplitude of acoustic signal340 exceeds S3 threshold 347 during S3 window 356. S3 window delay 357and the duration of S3 window 356 are programmed on a patient-by-patientbasis. In one embodiment, S3 window delay 357 is programmable between100 and 200 milliseconds. The duration of S3 window 356 is programmableto about 150 ms but is terminated by V event 349 of the next cardiaccycle it occurs before the end of the programmed duration.

As illustrated in FIG. 2, heart sound detector 105 includes a heart ratedetector 220, an S1 detector 222, an S2 detection module (including anS2 detection preparation module, an S2 window generator 225, an S2threshold generator 226, and an S2 detector 227), an S3 detection module(including an S3 detection preparation module, an S3 window generator230, an S3 threshold generator 231, and an S3 detector 232), and ameasurement module 234. Heart rate detector 220 detects a heart ratefrom the cardiac signal such as the signal received by cardiac signalinput 101.

S1 detector is required when S3 threshold 347 depends at least partiallyon the S1 amplitude or energy. It is also required when, as part of anoverall signal processing and analysis scheme, measurement related to S1are taken.

S2 detection preparation module 224, S2 window generator 225, S2threshold generator 226, and S2 detector 227 perform S2 detection. S2detection preparation module 224 is needed when acoustic signal 340needs to be further processed to facilitate an accurate S2 detection. Inone embodiment, S2 detection preparation module 224 includes anaveraging circuit that improves the signal-to-noise ratio of acousticsignal 340 by ensemble averaging. In one specific embodiment, theaveraging circuit aligns multiple segments of acoustic signal 340 by Vevent markers representing V event detection 349 on each segment. Thesegments of acoustic signal 340 to be included for the ensembleaveraging are selected from segments of acoustic signal 340 associatedwith consecutive cardiac cycles with a relatively constant heart rate(e.g., within about 10 to 20 beats per minute variation). S2 windowgenerator 225 includes an S2 window delay timer to time S2 window delay355 and an S2 window timer to time S2 window 354. V event detection 349(represented by such as an event marker include in the cardiac signal)triggers the S2 window delay timer to start timing S2 window delay 355.The expiration of S2 window delay 355 triggers the S2 window timer totime S2 window 354, during which S2 is being detected. S2 thresholdgenerator 226 generates S2 threshold 346 based on the amplitude of S1and/or the amplitude of S2. In one embodiment, S2 threshold generator226 dynamically adjusts S2 threshold 346 based on the amplitude of S1and/or the amplitude of S2 averaged over a moving window including aplurality of heart beats. S2 detector 227 includes a comparator tocompare acoustic signal 340 to S2 threshold 346, and detects anoccurrence of S2 when the amplitude of acoustic signal 340 exceeds S2threshold 346 during S2 window 354.

S3 detection preparation module 229, S3 window generator 230, S3threshold generator 231, and S3 detector 232 perform the S3 detection.S3 detection preparation module 229 is needed when acoustic signal 340needs to be further processed to facilitate an accurate S3 detection. Inone embodiment, S3 detection preparation module 229 includes theaveraging circuit of S2 detection preparation module 224, and theaveraged acoustic signal is used for both S2 and S3 detection. In onespecific embodiment, S3 detection preparation module 229 and S2detection preparation module 224 includes a single averaging circuit—thesame averaging circuit that performs the ensemble averaging discussedabove. In another embodiment, S3 detection preparation module 229includes an averaging circuit for the purpose of S3 detection only. Thisaveraging circuit improves the signal-to-noise ratio of acoustic signal340 for the S3 detection by an ensemble averaging process. The averagingcircuit aligns multiple segments of acoustic signal 340 by S2 markersrepresenting S2 detection 350 on each segment, where the S2 detection350 is resulted from S2 detection performed on acoustic signal 340before the ensemble averaging. The segments of acoustic signal 340 to beincluded for the ensemble averaging are selected from segments ofacoustic signal 340 associated with consecutive cardiac cycles with arelatively constant heart rate (e.g., within about 10 to 20 beats perminute variation). S3 window generator 230 includes an S3 window delaytimer to time S3 window delay 357 and an S2 window timer to time S3window 356. S3 detection 350 triggers the S3 window delay timer to starttiming S3 window delay 357. The expiration of S3 window delay 357triggers the S3 window timer to time S3 window 356, during which S3 isbeing detected. S3 threshold generator 231 generates S3 threshold 347.In one embodiment, S3 threshold generator 231 determines S3 threshold347 based on one or more of an S1 amplitude and an S2 amplitude. In onespecific embodiment, S3 threshold generator 231 determines S3 threshold347 as a percentage of the S2 amplitude. In another embodiment, S3threshold generator 231 determines S3 threshold 347 based on a totalacoustic energy in one cardiac cycle. The purpose is to normalize S3threshold 347 by the total acoustic energy, such that the S3 detectionremains accurate when external variables causes the amplitude ofacoustic signal 340 to shift. The total acoustic energy is calculated byintegrating acoustic signal 340 over one cardiac cycle or adding theestimated energies of S1, S2, and S3 (if present). In one specificexample, S3 threshold generator 231 determines S3 threshold 347 based onat least a mean and a standard deviation of the total acoustic energy inthe cardiac cycle. In another embodiment, S3 threshold generator 231determines S3 threshold 347 based on a total acoustic energy duringsystole of one cardiac cycle. The total acoustic energy is calculated byintegrating acoustic signal 340 over the time interval between V eventdetection 349 and the beginning of S3 window 356 or adding the estimatedenergies of S1 and S2. In one specific embodiment, S3 thresholdgenerator 231 determines S3 threshold 347 based on at least a mean and astandard deviation of the total acoustic energy during systole of thecardiac cycle. In another embodiment, S3 threshold generator 231determines S3 threshold 347 based on a temporal average of one or moreof the S1 amplitude, the S2 amplitude, and the total acoustic energy inone cardiac cycle. In another embodiment, S3 threshold generator 231determines S3 threshold 347 based on an estimated background sound level345 (μ_(B)) measured during a background estimate period 353, which is apredetermined period between S1 and S2, as illustrated in FIG. 3.Background estimate period 353 is centered between T_(S1max) andT_(S2min), where T_(S1max) is the latest point in time where S1 energyis expected, and is T_(S2min) is the point in time where S2 window 354begins. In one embodiment, T_(S1max) is empirically estimated based on apatient population. In one embodiment, T_(S1max) is in a range of about100 to 200 millisecond after V event detection 349. In one embodiment,the duration of background estimate period 353 substantially equals tothe time interval between T_(S1max) and T_(S2min). In anotherembodiment, the duration of background estimate period 353 is shorterthan the time interval between T_(S1max) and T_(S2min). In anotherembodiment, the duration of background estimate period 353 is set to beequal to the time interval between T_(S1max) and T_(S2min) but subjectedto a maximum duration and a minimum duration. The duration of backgroundestimate period 353 is set to the maximum duration if the time intervalbetween T_(S1max) and T_(S2min) is longer than the maximum duration andto the minimum duration if the time interval between T_(S1max) andT_(S2min) is shorter than the minimum duration. In one embodiment, S3threshold generator 231 scales background estimate period 353 for thedetected heart rate. S3 threshold 347 is a function of μ_(B). In onespecific embodiment, S3 threshold generator 231 sets and dynamicallyadjusts S3 threshold 347 to μ_(B)*K, where K is a programmable constant.In one embodiment, K is a constant determined, and can be lateradjusted, by a user. In one embodiment, K is a constant false alarm rate(CFAR) as known in the art of signal processing. S3 detector 232includes a comparator to compare acoustic signal 340 to S3 threshold347, and detects an occurrence of S3 when the amplitude of acousticsignal 340 exceeds S3 threshold 347 during S3 window 356.

Measurement module 234 measures parameters from detected S1, S2 and/orS3. In one embodiment, the measured parameters are used to set andadjust S2 threshold 346 and/or S3 threshold 347. In another embodiment,the measured parameters are used for various cardiac functional analysisbased on or related to heart sounds. In one embodiment, measurementmodule 234 includes an amplitude detector to detect amplitudes of S1,S2, and/or S3. In one further embodiment, measurement module 234includes an energy detector to detect energies associated with S1, S2,and/or S3. Examples of other parameters measured by measurement module234 include, but are not limited to, duration of any type heart sound,fundamental and/or harmonic frequencies of any heart sound, timeinterval between any two heart sounds, and time interval between anintrinsic or paced cardiac event detected from the electrogram and aheart sound detected from the acoustic signal.

FIG. 4 is a flow chart illustrating an embodiment of a method fortrending the S3 index. In one embodiment, the method is performed usingheart sound processing system 100. The method includes acquiring cardiacand acoustic signals at 400, timing S3 index trending at 420, andtrending the S3 index at 440.

Cardiac and acoustic signals required for trending the S3 index aresensed and preprocessed at 400. At 402, an acoustic signal is sensed. Inone embodiment, this includes sensing an audio signal generated from aheart using a microphone placed in or near the heart. In anotherembodiment, this includes sensing a mechanical vibration of the heartusing an accelerometer placed in or near the heart. The acoustic signalis preprocessed at 404. In one embodiment, this includes performingenvelope detection, i.e., rectifying and low-pass filtering the sensedacoustic signal. One example of a resultant preprocessed acoustic signalis illustrated as acoustic signal 340 in FIG. 3. At 406, at least oneelectrogram is sensed. In one embodiment, this includes sensing aventricular electrogram with at least one electrode placed a ventricularchamber of the heart to sense ventricular events. A cardiac signal isproduced at 408. In one embodiment, this includes detecting cardiacevents indicated in the sensed electrogram and producing event markerseach indicative of an occurrence of the detected cardiac events,including its type and timing. In one specific example, the cardiacsignal includes ventricular event markers representing sensedventricular contractions and/or deliveries of ventricular pacing pulses.

The heart sound analysis producing the trend of the S3 index is aperiodic process timed at 420. The timing includes timing measurementperiods each including a plurality of measurement sessions. In oneembodiment, the measurement period is a predetermined time perioddefined as a number of hours, days, weeks, etc. Each measurement periodincludes a plurality of prescheduled measurement sessions. In oneembodiment, a physical activity level is detected at 422. The physicalactivity level indicates a person's gross bodily movements that mayinterfere with the sensing of the acoustic signal. A permission signalis issued at 424 to allow the start of a measurement session if thephysical activity level is below a threshold level indicating that theperson is resting. If the permission signal is present when ameasurement session is scheduled to begin, a measurement session istimed at 426. In one embodiment, the measurement session is timed as apredetermined period of time. In another embodiment, the measurementsession is timed by counting a predetermined number of heart beats. Ifthe permission signal is absent when a measurement session is scheduledto begin, the measurement session is postponed for a predeterminedperiod of time or until the physical activity level falls below thethreshold level. In one embodiment, timing S3 index trending at 420 alsoincludes timing the acquisition of the cardiac and acoustic signals.

A trend of the S3 index is produced at 440. The cardiac signal producedat 408 and the acoustic signal preprocessed at 404 are received at 442.Heart beats are counted at 444 for each measurement session or period,based on one type of cardiac events included in the cardiac signal. Inone embodiment, one heart beat is counted for each ventricular event.Occurrences of S3 are detected at 446. The S3 beats are counted at 448for each measurement session or period. The S3 index is calculated at450, as the ratio of the number of S3 beats to the number of total heartbeats counted during the measurement session or period. In oneembodiment, the ratio is expressed as a percentage. The trend of the S3index is produced at 452. In one embodiment, the trend of the S3 indexis a moving S3 index calculated for the measurement periods. In anotherembodiment, the trend of the S3 index is a moving average of the S3index calculated for the measurement sessions over the measurementperiods. In one embodiment, trend of the S3 index is presented as a plotof S3 indices over a measured period or another predetermined period. Inone specific embodiment, trend of the S3 index is presented as a plot ofdaily S3 indices.

FIG. 5 is a flow chart illustrating an embodiment of a method fordetecting S3. In one embodiment, the method is performed using heartsound detector 105. The method includes acquiring cardiac and acousticsignals at 500, detecting heart sounds at 520, and measuring parametersrelated to the detected heart sounds at 550.

Cardiac and acoustic signals are sensed and preprocessed at 500. At 502,an acoustic signal is sensed. In one embodiment, this includes sensingan audio signal generated from a heart using a microphone placed in ornear the heart. In another embodiment, this includes sensing amechanical vibration of the heart using an accelerometer placed in ornear the heart. The acoustic signal is envelope-detected, i.e.,rectified and low-pass filtered, at 504. One example of theenvelope-detected acoustic signal is illustrated as acoustic signal 340in FIG. 3. At 506, a ventricular electrogram is sensed. A cardiac signalis produced at 508. This includes detecting ventricular events,including sensed ventricular contractions and/or deliveries ofventricular pacing pulses, and generating ventricular event markersrepresenting the detected ventricular events. In one embodiment, whenthe S3 index trending method (illustrated in FIG. 4) employs the S3detection method (illustrated in FIG. 5), step 400 and step 500 includesubstantially the same steps.

Heart sounds including S1, S2, and/or S3 are detected at 520. Thecardiac signal and the acoustic signal are received at 522. S1 isdetected at 524. In one embodiment, detecting S1 includes comparing theamplitude of the preprocessed acoustic signal to an S1 threshold. An S2window is generated at 526. An S2 threshold is determined at 528. S2 isdetected at 530. In one embodiment, steps 526, 528, and 530 areperformed using the method discussed above with reference to FIG. 3. AnS3 window is generated at 532. An S3 threshold is determined at 534. S3is detected at 536. In one embodiment, steps 532, 534, and 536 areperformed using the method discussed above with reference to FIG. 3.

Parameters are measured from the detected heart sounds at 550. In oneembodiment, the parameters are used to determine the S1 threshold, theS2 threshold, and/or the S3 threshold. Amplitudes of S1, S2, and/or S3are measured at 552. In one embodiment, each amplitude is measured as anaverage of amplitudes of one type heart sound measured over a pluralityof heart beats. Energies associated with S1, S2, and/or S3 are measuredat 554. In one embodiment, each energy is measured as an average ofenergies associated with one type heart sound measured over a pluralityof heart beats. A total acoustic energy during a cardiac cycle ismeasured (and/or calculated) at 556. In one embodiment, the totalacoustic energy during the cardiac cycle is measured as an average overa plurality of cardiac cycles (heart beats). A total acoustic energyduring systole is measured (and/or calculated) at 558. In oneembodiment, the total acoustic energy during systole is measured as anaverage over a plurality of heart beats.

In one embodiment, one or more parameters measured at 550 are used foranalyzing cardiac conditions. One example of such parameter measurementand use is discussed in U.S. patent application Ser. No. 10/334,694,entitled “METHOD AND APPARATUS FOR MONITORING OF DIASTOLICHEMODYNAMICS,” filed Dec. 30, 2002, assigned to Cardiac Pacemakers,Inc., the specification of which is incorporated herein by reference inits entirety.

FIGS. 4 and 5 are for illustrative purposes and do not necessarilyreflect the exact temporal order by which the steps are performed inpractice. The actual order or sequence by which the steps are performeddepends on logic and design considerations.

FIG. 6 is an illustration of an embodiment of portions of a cardiacrhythm management system 699 incorporating heart sound processing system100, and portions of an environment in which it is used. In oneembodiment, cardiac rhythm management system 699 includes an implantablesystem 665, an external system 670, and a telemetry link 669 providingfor communication between implantable system 665 and external system670.

Implantable system 665 includes an implantable device 667 and a leadsystem 668. Implantable device 667 is implanted within a patient's body662 and electrically connected to the patient's heart 661 via leadsystem 668. Examples of implantable device 667 include pacemakers,cardioverter/defibrillators, pacemaker/defibrillators, cardiacresynchronization devices, cardiac remodeling control devices, and drugdelivery devices.

In one embodiment, external system 670 includes an external device 671in proximity of implantable device 667, a remote device 673 in arelatively distant location, and a telecommunication system 672 linkingexternal device 671 and remote device 673. An example of such anexternal system includes an advanced patient management system discussedin U.S. patent application Ser. No. 10/323,604, entitled “ADVANCEDPATIENT MANAGEMENT FOR DEFINING, IDENTIFYING AND USING PREDETERMINEDHEALTH-RELATED EVENTS,” filed on Dec. 18, 2002, published as US20040122484, assigned to Cardiac Pacemakers, Inc., the specification ofwhich is incorporated herein by reference in its entirety. In anotherembodiment, external system 670 includes an implantable medical deviceprogrammer.

In one embodiment, telemetry link 669 is an inductive telemetry link. Inan alternative embodiment, telemetry link 669 is a far-fieldradio-frequency telemetry link. In one embodiment, telemetry link 669provides for data transmission from implantable device 667 to externaldevice 671. This may include, for example, transmitting real-timephysiological data acquired by implantable device 667, extractingphysiological data acquired by and stored in implantable device 667,extracting therapy history data stored in implantable device 667, andextracting data indicating an operational status of implantable device667 (e.g., battery status and lead impedance). In a further embodiment,telemetry link 669 provides for data transmission from external device671 to implantable device 667. This may include, for example,programming implantable device 667 to acquire physiological data,programming implantable device 667 to perform at least oneself-diagnostic test (such as for a device operational status), andprogramming implantable device 667 to deliver at least one therapy.

In one embodiment, programming implantable device 667 includes sendingtherapy parameters to implantable device 667. The therapy parametersprovide an improved hemodynamic performance for a patient by deliveringcardiac pacing pulses to the patient's heart. In one embodiment, thetherapy parameters providing for the improved hemodynamic performanceare determined by monitoring one or more ventricular diastolichemodynamics as indicated by parameters related to heart sounds. Suchparameters indicate the heart's mechanical activities andelectromechanical association. In one specific embodiment, theparameters related to heart sounds are measured by heart soundprocessing system 100, as discussed above with reference to FIGS. 1-3.

FIG. 7 is a block diagram illustrating a specific embodiment of portionsof cardiac rhythm management system 699. Cardiac rhythm managementsystem 699 includes implantable system 665, external system 670, and atelemetry link 669 wirelessly coupling these two systems. Heart soundprocessing system 100 is included in implantable system 665 and/orexternal system 670. The actual distribution of heart sound processingsystem 100 in implantable system 665 and/or external system 670 dependson practical considerations such as power and size constraints of animplantable device, telemetry capability, and systemic factors such asefficiency of operation and convenience of use of cardiac rhythmmanagement system 699. In one embodiment, implantable system 665includes the entire heart sound processing system 100. In anotherembodiment, external system 670 includes the entire heart soundprocessing system 100. In another embodiment, implantable system 665includes portions of heart sound processing system 100 and externalsystem 670 includes the remaining portions of heart sound processingsystem 100.

Implantable system 665 includes implantable device 667 and lead system668. Lead system 668 electrically connects implantable device 667 toheart 661. In one embodiment, implantable device 667 includes ahermetically sealed can to house an implantable acoustic sensor 780, asensor processing circuit 781, a cardiac sensing circuit 784, a cardiacevent detector 785, an implant controller 782, an implant heart soundprocessor 786, and an implant telemetry module 775. In anotherembodiment, implantable acoustic sensor 780 is incorporated into a leadof lead system 668. Implantable acoustic sensor 780 senses the acousticsignal indicative of the heart's mechanical vibrations, i.e., heartsounds, of heart 661. In one embodiment, implantable acoustic sensor 780includes an accelerometer. In another embodiment, implantable acousticsensor 780 includes a microphone. Sensor processing circuit 781preconditions the sensed acoustic signal for further processing by heartsound processing system 100. In one embodiment, the acoustic signal isenvelope detected. Sensor processing circuit 781 includes a rectifierand a low-pass filter to perform the envelope detection. Cardiac sensingcircuit 784 includes sense amplifiers and filters to sense one or moreelectrograms. In one embodiment, cardiac sensing circuit 784 senses atleast a ventricular electrogram indicative of ventricular eventsincluding intrinsic ventricular contractions and ventricular pacingpulse deliveries. Cardiac event detector 785 detects predetermined typesof cardiac events. In one embodiment, cardiac event detector 785 detectsthe ventricular events including the intrinsic ventricular contractionsand the ventricular pacing pulse deliveries. In one embodiment, cardiacevent detector 785 generates event markers to represent the detectedevents. The event markers each indicate an occurrence, a timing of theoccurrence, and a type of the detected event represented. Implantcontroller 782 controls the overall operation of implantable system 665.In one embodiment, implant heart sound processor 786 includes at least aportion of heart sound processing system 100. In one specificembodiment, implant heart sound processor 786 includes substantially theentire heart sound processing system 100. Implant telemetry module 775is the portion of the circuit supporting telemetry link 669 that residesin implantable system 665.

External system 670 includes an external telemetry module 776 and anexternal heart sound module 778. External telemetry module 776 is theportion of the circuit supporting telemetry link 669 that resides inexternal system 670. External heart sound module 778 includes anexternal heart sound processor 790. In one embodiment, external heartsound processor 790 includes at least a portion of heart soundprocessing system 100. In one specific embodiment, external heart soundprocessor 790 includes substantially the entire heart sound processingsystem 100. In one embodiment, external heart sound processor 790 issubstantially included in external device 671 of external system 670. Inanother embodiment, external heart sound processor 790 is substantiallyincluded in remote device 673 of external system 670. In anotherembodiment, external heart sound processor 790 is distributed in bothexternal device 671 and remote device 673.

In one embodiment, in addition to the functions of external heart soundprocessor 790, external heart sound module 778 analyzes parametersderived from detected cardiac events and heart sounds. Examples of suchanalyses are discussed in U.S. patent application Ser. No. 10/307,896,“PHONOCARDIOGRAPHIC IMAGE-BASED ATRIOVENTRICULAR DELAY OPTIMIZATION.”filed Dec. 12, 2002, and U.S. patent application Ser. No. 10/334,694,entitled “METHOD AND APPARATUS FOR MONITORING OF DIASTOLICHEMODYNAMICS,” filed Dec. 30, 2002, now issued as U.S. Pat. No.7,972,275 both assigned to Cardiac Pacemakers, Inc., the specificationsof which are incorporated herein by reference in their entirety.

It is to be understood that the above detailed description is intendedto be illustrative, and not restrictive. For example, heart soundprocessing system 100 may be incorporated into any implanted or externalmedical device providing for ECG and heart sound monitoring. Otherembodiments will be apparent to those of skill in the art upon readingand understanding the above description. The scope of the inventionshould, therefore, be determined with reference to the appended claims,along with the full scope of equivalents to which such claims areentitled.

1. A heart sound detection system, comprising: an acoustic signal inputadapted to receive an acoustic signal indicative of heart soundsincluding second heart sounds (S2) and third heart sounds (S3); and aheart sound detector coupled to the acoustic signal input, the heartsound detector including: an S2 detector adapted to detect S2 using theacoustic signal; an S3 window generator adapted to generate an S3 windowin response to each detection of S2; an S3 detector adapted to detect S3using the acoustic signal and an S3 threshold during the S3 windows; ameasurement module adapted to detect an acoustic energy using theacoustic signal; and an S3 threshold generator adapted to determine theS3 threshold using the acoustic energy, wherein the S3 thresholdgenerator is adapted to normalize the S3 threshold using the acousticenergy.
 2. The system of claim 1, wherein the S3 window generator isadapted to generate the S3 window after a programmed delay starting witheach of the detected S2.
 3. The system of claim 2, wherein themeasurement module is adapted to detect a total acoustic energy during acardiac cycle, and the S3 threshold generator is adapted to dynamicallyadjust the S3 threshold using the total acoustic energy detected duringthe cardiac cycle.
 4. The system of claim 3, wherein the S3 thresholdgenerator is adapted to dynamically adjust the S3 threshold using a meanand a standard deviation of the total acoustic energy during the cardiaccycle.
 5. The system of claim 2, wherein the measurement module isadapted to detect a total acoustic energy during systole of a cardiaccycle, and the S3 threshold generator is adapted to dynamically adjustthe S3 threshold using the total acoustic energy during systole of thecardiac cycle.
 6. The system of claim 5, wherein the S3 thresholdgenerator is adapted to dynamically adjust the S3 threshold using a meanand a standard deviation of the total acoustic energy during the systoleof the cardiac cycle.
 7. A heart sound detection method, comprising:receiving an acoustic signal indicative of heart sounds including secondheart sounds (S2) and third heard sounds (S3); detecting S2 using theacoustic signal; generating an S3 window following each of the detectedS2; detecting S3 during the S3 windows by comparing the acoustic signalto an S3 threshold; detecting an acoustic energy using the acousticsignal; and dynamically adjusting the S3 threshold using the acousticenergy, including normalizing the S3 threshold using the acousticenergy.
 8. The method of claim 7, wherein generating the S3 windowcomprises generating the S3 window after a programmed delay startingwith each of the detected S2.
 9. The method of claim 8, comprising:receiving a cardiac signal indicative of ventricular events; detectingventricular events from the cardiac signal; and terminating the S3window if one of the ventricular events occurs during the S3 window. 10.The method of claim 8, comprising determining the S3 threshold using atleast a mean and a standard deviation of the acoustic energy.
 11. Themethod of claim 8, wherein detecting the acoustic energy comprisesdetecting a total acoustic energy during a cardiac cycle as the acousticenergy.
 12. The method of claim 8, wherein detecting the acoustic energycomprises detecting a total acoustic energy during systole of a cardiaccycle as the acoustic energy.
 13. A heart sound detection system,comprising: an acoustic signal input to receive an acoustic signalindicative of heart sounds including first heart sounds (S1), secondheart sounds (S2), and third heart sounds (S3); and a heart sounddetector coupled to the acoustic signal input, the heart sound detectorincluding: an S2 detector adapted to detect S2 using the acousticsignal; an S3 detection preparation module adapted to align segments ofthe acoustic signal using the detected S2 and ensemble average theacoustic signal using the aligned segments; an S3 window generatoradapted to generate an S3 window in response to each detection of S2; anS3 detector adapted to detect S3 during the S3 windows using theaveraged acoustic signal and an S3 threshold; and an S3 thresholdgenerator adapted to determine the S3 threshold using an estimatedbackground sound level (μ_(B)) measured during a period between anoccurrence of S1 and an adjacent occurrence of S2.
 14. The system ofclaim 13, wherein the S3 threshold generator is adapted to dynamicallyadjust the S3 threshold using μ_(B).
 15. The system of claim 14, whereinthe S3 threshold generator is adapted to calculate the S3 threshold asμ_(B)*K, where K is an adjustable multiplier.
 16. The system of claim15, wherein the S3 threshold generator is adapted to scale the periodbetween the occurrence of S1 and the adjacent occurrence of S2 for aheart rate.
 17. The system of claim 16, comprising: a cardiac signalinput adapted to receive a cardiac signal indicative of ventricularevents; and an S2 window generator adapted to generate an S2 window inresponse to the detection of each of the ventricular events, and whereinthe S2 detector is adapted to detect S2 during the S2 windows.
 18. Thesystem of claim 17, wherein the S3 window generator is adapted togenerate the S3 window after a second programmed delay starting witheach of the detected S2.
 19. The system of claim 13, wherein the S3threshold generator is adapted to determine the S3 threshold using theμ_(B) measured during a period set to be substantially equal to a timeinterval between a latest point in time where S1 energy is expected anda point in time where the S2 window begins and to be subjected to amaximum duration and a minimum duration.
 20. The system of claim 13,wherein the S3 threshold generator is adapted to dynamically adjustingthe S3 threshold using the μ_(B) measured during a period centeredbetween a latest point in time where S1 energy is expected and a pointin time where the S2 window begins, wherein the latest point in timewhere S1 energy is expected is empirically estimated based on a patientpopulation.
 21. A heart sound detection method, comprising: receiving anacoustic signal indicative of heart sounds including first heart sounds(S1), second heart sounds (S2), and third heart sounds (S3); detectingS2 using the acoustic signal; aligning segments of the acoustic signalusing the detected S2; ensemble averaging the acoustic signal using thealigned segments; generating an S3 window following each of the detectedS2; detecting S3 during the S3 windows by comparing the ensembleaveraged acoustic signal to a dynamically adjustable S3 threshold; anddynamically adjusting the S3 threshold using an estimated backgroundsound level (μ_(B)) measured during a period between an occurrence of S1and an adjacent occurrence of S2.
 22. The method of claim 21, whereindynamically adjusting the S3 threshold comprises calculating the S3threshold as μ_(B)*K, where K is an adjustable.
 23. The method of claim22, comprising: receiving a cardiac signal indicative of ventricularevents; detecting a heart rate using the cardiac signal; and scaling theperiod between the occurrence of S1 and the adjacent occurrence of S2for the heart rate.
 24. The method of claim 23, comprising: detectingventricular events from the cardiac signal; and terminating the S3window if one of the ventricular events occurs during the S3 window. 25.The method of claim 21, comprising: receiving a cardiac signalindicative of ventricular events; and generating an S2 window inresponse to the detection of each of the ventricular events, and whereindetecting S2 comprises detecting S2 during the S2 windows.
 26. Themethod of claim 25, wherein generating the S3 window comprisesgenerating the S3 window after a programmed delay starting with each ofthe detected S2.
 27. The method of claim 21, wherein dynamicallyadjusting the S3 threshold comprises dynamically adjusting the S3threshold using the μ_(B) measured during a period set to besubstantially equal to a time interval between a latest point in timewhere S1 energy is expected and a point in time where the S2 windowbegins and to be subjected to a maximum duration and a minimum duration.28. The system of claim 21, wherein dynamically adjusting the S3threshold comprises: dynamically adjusting the S3 threshold using themeasured during a period centered between a latest point in time whereS1 energy is expected and a point in time where the S2 window begins;and estimating the latest point in time where S1 energy is expectedempirically based on a patient population.